README.md

# MASSCAN: Mass IP port scanner

This is the fastest Internet port scanner. It can scan the entire Internet
in under 6 minutes, transmitting 10 million packets per second.

It produces results similar to `nmap`, the most famous port scanner.
Internally, it operates more like `scanrand`, `unicornscan`, and `ZMap`, using
asynchronous transmission. The major difference is that it's faster than these
other scanners. In addition, it's more flexible, allowing arbitrary address
ranges and port ranges.


# Building

On Debian/Ubuntu, it goes something like this:

	$ git clone https://github.com/robertdavidgraham/masscan
	$ cd masscan
	$ sudo apt-get install libpcap-dev
	$ make

This puts the program in the `masscan/bin` subdirectory. You'll have to
manually copy it to something like `/usr/local/bin` if you want to
install it elsewhere on the system.

While Linux is the primary target platform, the code runs well on many other
systems. Here's some additional build info:
* Windows w/ Visual Studio: use the VS10 project
* Windows w/ MingGW: just type `make`
* Windows w/ cygwin: won't work
* Mac OS X /w XCode: use the XCode4 project
* Mac OS X /w cmdline: just type `make`
* FreeBSD: type `gmake`
* other: I don't know, don't care


## PF_RING

To get beyond 2 million packets/second, you need an Intel 10-gbps Ethernet
adapter and a special driver known as "PF_RING DNA" from http://www.netop.org.
Masscan doesn't need to be rebuilt in order to use PF_RING. To use PF_RING,
you need to build the following components:
* `libpfring.so` (installed in /usr/lib/libpfring.so)
* `pf_ring.ko` (their kernel driver)
* `ixgbe.ko` (their version of the Intel 10-gbps Ethernet driver)

You don't need to build their version of `libpcap.so`.

When Masscan detects that an adapter is named something like `dna0` instead
of something like `eth0`, it'll automatically switch to PF_RING mode.


## Regression testing

The project contains a built-in self-test:

	$ make regress
	bin/masscan --regress
	selftest: success!

This tests a lot of tricky bits of the code. You should do this after building.


## Performance testing

To test performance, run something like the following:

	$ bin/masscan 0.0.0.0/4 -p80 --rate 100000000 --router-mac 66-55-44-33-22-11

The bogus `--router-mac` keeps packets on the local network segments so that
they won't go out to the Internet.

You can also test in "offline" mode, which is how fast the program runs
without the transmit overhead:

	$ bin/masscan 0.0.0.0/4 -p80 --rate 100000000 --offline
    
This second benchmark shows roughly how fast the program would run if it were
using PF_RING, which has near zero overhead.


# Usage

Usage is similar to `nmap`. To scan a network segment for some ports:

	# masscan -p80,8000-8100 10.0.0.0/8

This will:
* scan the 10.x.x.x subnet, all 16 million addresses
* scans port 80 and the range 8000 to 8100, or 102 addresses total
* print output to <stdout> that can be redirected to a file

To see the complete list of options, use the `--echo` feature. This
dumps the current configuration and exits. This ouput can be used as input back
into the program:

	# masscan -p80,8000-8100 10.0.0.0/8 --echo > xxx.conf
	# masscan -c xxx.conf --rate 1000


## How to scan the entire Internet

While useful for smaller, internal networks, the program is designed really
with the entire Internet in mind. It might look something like this:

	# masscan 0.0.0.0/0 -p0-65535

Scanning the entire Internet is bad. For one thing, parts of the Internet react
badly to being scanned. For another thing, some sites track scans and add you
to a ban list, which will get you firewalled from useful parts of the Internet.
Therefore, you want to exlude a lot of ranges. To blacklist or exclude ranges,
you want to use the following syntax:

	# masscan 0.0.0.0/0 -p0-65535 --excludefile exclude.txt

This just prints the results to the command-line. You probably want them
saved to a file instead. Therefore, you want something like:

	# masscan 0.0.0.0/0 -p0-65535 -oX scan.xml

This saves the results in an XML file, allowing you to easily dump the
results in a database or something.

But, this only goes at the default rate of 100 packets/second, which will
take forever to scan the Internet. You need to speed it up as so:

	# masscan 0.0.0.0/0 -p0-65535 --max-rate 100000

This increases the rate to 100,000 packets/second, which will scan the
entire Internet (minus excludes) in about 10 hours per port (or 655,360 hours
if scanning all ports).

The thing to notice about this command-line is that these are all `nmap`
compatible options. In addition, "invisible" options compatible with `nmap`
are also set for you: `-sS -Pn -n --randomize-hosts --send-eth`. Likewise,
the format of the XML file is inspired by `nmap`. There are, of course, a
lot of differences, because the *asynchronous* nature of the program
leads to a fundamentally different approach to the problem.

The above command-line is a bit cumbersome. Instead of putting everything
on the command-line, it can be stored in a file instead. The above settings
would look like this:

	# My Scan
	rate =  100000.00
	output-format = xml
	output-status = all
	output-filename = scan.xml
	ports = 0-65535
	range = 0.0.0.0-255.255.255.255
	excludefile = exclude.txt

To use this configuration file, use the `-c`:

	# masscan -c myscan.conf

This also makes things easier when you repeat a scan.

By default, masscan first loads the configuration file 
`/etc/masscan/masscan.conf`. Any later configuration parameters override what's
in this default configuration file. That's where I put my "excludefile" 
parameter, so that I don't ever forget it. It just works automatically.

## Getting output

The are two primary formats for output. The first is XML, which products
fairly large files, but is easy to import into anything. Just use the
parameter `-oX <filename>`. Or, use the parameters `--output-format xml` and
`--output-filename <filename>`.

The second is the binary format. This produces much smaller files, so that
when I scan the Internet my disk doesn't fill up. They need to be parsed,
though. In the `util` subdirectory there is a program `scan2text.c` that will
scan in the binary format and produce text.


## Comparison with Nmap

Where reasonable, every effort has been taken to make the program familiar
to `nmap` users, even though it's fundamentally different. Two important
differences are:

* no default ports to scan, you must specify `-p <ports>`
* target hosts are IP addresses or simple ranges, not DNS names, nor 
  the funky subnet ranges `nmap` can use (like `10.0.0-255.0-255`).

You can think of `masscan` as having the following settings permanently
enabled:
* `-sS`: this does SYN scan only (currently, will change in future)
* `-Pn`: doesn't ping hosts first, which is fundamental to the async operation
* `-n`: no DNS resolution happens
* `--randomize-hosts`: scan completely randomized
* `--send-eth`: sends using raw `libpcap`

If you want a list of additional `nmap` compatible settings, use the following
command:

	# masscan --nmap


## Transmit rate (IMPORTANT!!)

This program spews out packets very fast. On Windows, or from VMs,
it can do 300,000 packets/second. On a Linux (no virtualization) it'll
do 1.6 million packets-per-second. That's fast enough to melt most networks.

Note that it'll only melt your own network. It randomizes the target
IP addresses so that it shouldn't overwhelm any distant network.

By default, the rate is set to 100 packets/second. To increase the rate to
a million use something like `--rate 1000000`.



# Design

This section describes the major design issues of the program.

## Code Layout

The file `main.c` contains the `main()` function, as you'd expect. It also
contains the `transmit_thread()` and `receive_thread()` functions. These
functions have been deliberately flattened and heavily commented so that you
can read the design of the program simply by stepping line-by-line through
each of these.

## Asynchronous

This is an *asynchronous* design. In other words, it is to `nmap` what
the `nginx` web-server is to `Apache`. It has separate transmit and receive
threads that are largely independent from each other. It's the same sort of
design found in `scanrand`, `unicornscan`, and `ZMap`.

Because it's asynchronous, it runs as fast as the underlying packet transmit
allows.


## Randomization

A key difference between Masscan and other scanners is the way it randomizes
targets.

The fundamental principle is to have a single index variable that starts at
zero and is incremented by one for every probe. In C code, this is expressed
as:

    for (i = 0; i < range; i++) {
        scan(i);
    }

We have to translate the index into an IP address. Let's say that you want to
scan all "private" IP addresses. That would be the table of ranges like:
    
    192.168.0.0/16
    10.0.0.0/8
    172.16.0.0/20

In this example, the first 64k indexes are appended to 192.168.x.x to form
the target address. Then, the next 16-million are appenedd to 10.x.x.x.
The remaining indexes in the range are applied to 172.16.x.x.

In this example, we only have three ranges. When scanning the entire Internet,
we have in practice more than 100 ranges. That's because you have to blacklist
or exlude a lot of sub-ranges. This chops up the desired range into hundreds
of smaller ranges.

This leads to one of the slowest parts of the code. We transmit 10 million
packets per second, and have to convert an index variable to an IP address
for each and every probe. We solve this by doing a "binary search" in a small
amount of memory. At this packet rate, cache efficiencies start to dominate
over algorithm efficiencies. There are a lot of more efficient techniques in
theory, but they all require so much memory as to be slower in practice.

We call the function that translates from an index into an IP address
the `pick()` function. In use, it looks like:

    for (i = 0; i < range; i++) {
        ip = pick(addresses, i);
        scan(ip);
    }

Masscan supports not only IP address ranges, but also port ranges. This means
we need to pick from the index variable both an IP address and a port. This
is fairly straight forward:

    range = ip_count * port_count;
    for (i = 0; i < range; i++) {
        ip   = pick(addresses, i / port_count);
        port = pick(ports,     i % port_count);
        scan(ip, port);
    }

This leads to another expensive part of the code. The division/modulus
instructions are around 90 clock cycles, or 30 nanoseconds, on x86 CPUs. When
transmitting at a rate of 10 million packets/second, we have only
100 nanoseconds per packet. I see now way to optimize this any better. Luckily,
though, two such operations can be executed simultaneously, so doing two 
of these as shown above is no more expesive than doing one.

There are actually some easy optimizations for the above performance problems,
but they all rely upon `i++`, the fact that the index variable increases one
by one through the scan. Actually, we need to randomize this variable. We
need to randomize the order of IP addresses that we scan or we'll blast the
heck out of target networks that aren't built for this level of speed. We 
need to spread our traffic evenly over the target.

The way we randomize is simply by encrypting the index variable. By definition,
encryption is random, and creates a 1-to-1 mapping between the original index
variable and the output. This means that while we linearly go through the
range, the output IP addresse are completely random. In code, this looks like:

    range = ip_count * port_count;
    for (i = 0; i < range; i++) {
        x = encrypt(i);
        ip   = pick(addresses, x / port_count);
        port = pick(ports,     x % port_count);
        scan(ip, port);
    }

This also has a major cost. Since the range is an unpredictable size instead
of a nice even power of 2, we can't use cheap binary techniques like
AND (&) and XOR (^). Instead, we have to use expensive operations like 
MODULUS (%). In my current benchmarks, it's taking 40 nanoseconds to
encrypt the variable.

This architecture allows for lots of cool features. For example, it supports
"shards". You can setup 5 machines each doing a fifth of the scan, or
`range / shard_count`. Shards can be multiple machines, or simply multiple
network adapters on the same machine, or even (if you want) multiple IP
source addresses on the same network adapter

Or, you can use a 'seed' or 'key' to the encryption function, so that you get
a different order each time you scan, like `x = encrypt(seed, i)`.

We can also pause the scan by exiting out of the program, and simply
remembering the current value of `i`, and restart it later. I do that a lot
during development. I see something going wrong with my Internet scan, so
I hit <ctrl-c> to stop the scan, then restart it after I've fixed the bug.

Another feature is retransmits/retries. Packets sometimes get dropped on the
Internet, so you can send two packets back-to-back. However, something that
drops one packet may drop the immediately following packet. Therefore, you
want to send the copy about 1 second apart. This is simple. We already have
a 'rate' variable, which is the number of packets-per-second rate we are
transmitting at, so the retransmit function is simply to use `i + rate`
as the index. One of these days I'm going to do a study of the Internet,
and differentiate "back-to-back", "1 second", "10 second", and "1 minute"
retransmits this way in order to see if there is any difference in what
gets dropped.



## C10 Scalability

The asynchronous technique is known as a solution to the "c10k problem".
Masscan is designed for the next level of scalability, the "C10M problem".

The C10M solution is to bypass the kernel. There are three primary kernel
bypasses in Masscan:
* custom network driver
* user-mode TCP stack
* user-mode synchronization

Masscan can use the PF_RING DNA driver. This driver DMA's packets directly
from user-mode memory to the network driver with zero kernel involvement.
That allows software, even with a slow CPU, to transmit packets at the maximum
rate the hardware allows. If you put 8 10-gbps network cards in a computer,
this means it could transmit at 100-million packets/second.

Masscan has it's own built-in TCP stack for grabbing banners from TCP
connections. It means it can easily support 10 million concurrent TCP
connections, assuming of course that the computer has enough memory.

Masscan has no "mutex". Modern mutexes (aka. futexes) are mostly user-mode,
but they have two problems. The first problem is that they cause cache-lines
to bounce quickly back-and-forth between CPUs. The second is that when there
is contention, they'll do a system call into the kernel, which kills
performance. Mutexes on the fast path of a program severely limits scalability.
Instead, Masscan uses "rings" to synchronize things, such as when the
user-mode TCP stack in the receive thread needs to transmit a packet without
interferring with the transmit thread.


## Portability

The code runs well on Linux, Windows, and Mac OS X. All the importnat bits are
in standard C (C90). It therefore compiles on Visual Studio with Microsoft's
compiler, the Clang/LLVM compiler on Mac OS X, and GCC on Linux.

Windows and Macs aren't tuned for packet transmit, and get only about 300,000
packets-per-second whereas Linux can do 1,500,000 packets/second. That's
probably faster than you want anyway.


## Safe code

A bounty is offered for vulnerabilities, see the VULNINFO.md file for more
information.

This project uses safe functions like `strcpy_s()` instead of unsafe functions
like `strcpy()`.

This project as automated unit regression tests (`make regress`).


## Compatibility

A lot of effort has been made in make the input/output look like `nmap`, which
everyone who does port scans is (or should be) familiar with.


# Authors

This tool created by Robert Graham:
email: robert_david_graham@yahoo.com
twitter: @ErrataRob